Adaptation of the amplitude of saccadic eye movements is necessary so that saccadic accuracy can be maintained throughout life despite the changes caused by development, injury and aging. The long-term objective of this grant is to study the mechanism and site(s) of saccadic amplitude adaptation in non-human primates. We will address this issue in several ways. First, we will estimate the level of the saccadic system at which adaptation takes place by determining whether behavioral adaptation of reactive (more simple) saccades transfers to higher-order (more "cognitive") saccades. Amplitude adaptation will be produced by requiring monkeys to track a stepping target that is jumped forward or backward during a targeting saccade, so that the adaptation mechanism is deceived into thinking that the saccade is in error. Over approximately 1000 such deceptions, monkeys gradually reduce this error by adjusting saccade amplitude. If adaptation of a reactive saccade does not transfer to a higher-order saccade, they must have different sites for saccadic plasticity. Second, we will record from the oculomotor cerebellar vermis during behavioral adaptation and assess the associated changes in simple spike and climbing fiber activity in Purkinje cells. We expect changes in neuronal firing that will indicate how the saccadic error delivered by the climbing fibers shapes the simple spike firing of Purkinje cells. Third, during behavioral adaptation we also will examine the change in activity of cells in the caudal fastigial nucleus (CFN), which receives direct inhibition from the vermal Purkinje cells and, in turn, projects directly to the premotor brain stem generator of saccadic eye movements. We expect that changes in firing of CFN cells will be appropriate to effect downstream structures to alter saccade amplitude. Because of the remarkable similarities of simian and human saccadic behavior, the results of these experiments should have considerable relevance in the diagnosis, treatment and rehabilitation of human patients with chronic saccadic disorders.